1. Field of the Invention
The present invention relates to an image forming apparatus having a toner supplying device for supplying a developer supplement (a toner, a premix toner including a toner and a carrier or the like) including at least a toner from a toner container to a developing device.
2. Discussion of the Background
Image forming apparatus typically use a toner supplying device for supplying a developer supplement (a toner, a premix toner including a carrier and a toner or the like) including at least a toner from a toner container to a developer containing portion of a developing device of the apparatus. Japanese patent 3,917,761 (i.e., published unexamined Japanese patent application No. 2000-47465) and published unexamined Japanese patent application No. 2004-226524 (which corresponds to U.S. Pat. No. 7,123,865) have disclosed toner supplying devices which supply a toner using a screw pump. Such toner supplying devices include a toner feeding passage, and a screw pump, which feeds the toner in the toner container to the developer containing portion by suction through the toner feeding passage.
Screw pumps typically include a stator which is made of an elastic material and which has a spiral groove on the inner surface thereof, and a rotor which is made of a metal and which has a spiral form and rotates inside the stator. In such screw pumps, a closed space is formed by the inner surface of the stator, the surface of the rotor and the contact portion between the rotor and the stator. When the rotor rotates, the rotor rubs the stator while a contact portion of the rotor moves relative to the stator, and therefore, the closed space moves in a direction parallel to the axis of the rotor (i.e., the rotor axis direction). The screw pumps have an opening at both the ends thereof in the rotor axis direction, i.e., a first opening present on the uppermost stream side relative to the moving direction of the closed space, and a second opening present on the most downstream side. The first opening serves as a suction opening (i.e., entrance), and the second opening serves as an exhaust opening (i.e., an exit). The first opening is connected with a toner container via a feeding tube serving as a feeding passage. The second opening is connected with a developer containing portion of a developing device directly or via another feeding passage or a hopper.
When such a screw pump is set in a toner supplying device and then driven, a negative pressure is generated at the suction opening due to movement of the closed space caused by rotation of the rotor, and thereby the toner in a toner container is sucked, resulting in entering of the toner into the space between the stator and rotor of the screw pump. The toner thus entering into the screw pump is moved toward the exhaust opening due to movement of the closed space while contained in the closed space. The toner thus fed to the exhaust opening of the screw pump is supplied to a developer containing portion of a developing device directly or via a feeding passage or a hopper.
A toner supplying device having such a screw pump sucks and feeds a toner using the screw pump. Therefore, even when the feeding tube serving as a feeding passage is curved or raised at an acute angle, the toner can be stably fed. Accordingly, by using a screw pump, the positional flexibility of the toner supplying device and developing device can be enhanced, namely the positions of the toner supplying device and developing device can be changed relatively freely. In addition, the feeding tube can be arranged between parts set in the vicinity of the toner supplying device. Therefore, the image forming apparatus can be miniaturized.
On the other hand, recently image forming apparatus are requested to save energy. In order to save energy, fixing temperature in heat fixing devices used for the image forming apparatus tends to be decreased. Therefore, image forming apparatus using a low temperature fixable toner, which can be fixed at a relatively low fixing temperature, have been proposed.
Recent image forming apparatus typically have a function such that when the toner in the toner container thereof is exhausted, a message “toner is nearly empty” is displayed in the operation panel of the image forming apparatus. Several methods have been used for detecting that toner is nearly empty. One of the near-empty detection methods is that a property (such as the amount of toner present in a sub-hopper) influenced by the amount of toner fed by the screw pump is measured at the main body of the image forming apparatus to determine whether the toner in the toner container is nearly empty. Specifically, this near-empty detection method is such that when a sensor determines that the amount of toner present in the sub-hopper is smaller than a predetermined amount, a controller orders to perform a toner supplying operation. Even when the amount of toner in the sub-hopper does not reach to the predetermined amount even after a predetermined number of toner supplying operations are performed, the controller determines that the toner container is nearly empty.
There are image forming apparatus, which continue image forming operations even after a near-empty detection as long as predetermined conditions are satisfied (for example, conditions such that the number of copies produced thereafter is within a predetermined range, and/or the image forming time and/or the toner consumption thereafter are within predetermined ranges are satisfied). In this regard, the number of additional image forming operations is determined as the number of images which can be produced without any problems even when the toner is not supplied from the toner container. It is common for conventional image forming apparatus that even when a near-empty message is displayed, a small amount of toner is present in the toner container. Therefore, such image forming apparatus typically perform the toner supplying operation in such additional image forming operations to reduce the amount of the residual toner in the toner container.
However, when screw pumps feed a low temperature fixable toner, a problem in that the suction power of the screw pumps deteriorate tends to occur. The cause for the problem is considered as follows.
When a rotor is rotated wile rubbing a stator in a screw pump, heat is generated due to the friction between the rotor and the stator. In addition, the rotor and stator form a closed space, and therefore the thus generated heat does not easily escape from the screw pump, resulting in increase of the internal temperature of the screw pump. In this regard, when a considerable amount of toner is present in the toner container, increase of the internal temperature of the screw pump is prevented. This is because a sufficient amount of toner is sucked by the screw pump, and the toner in the screw pump absorbs the heat and is then discharged from the screw pump, resulting in escape of the frictional heat from the screw pump.
In contrast, when the toner present in the toner container is such a small amount that a near-empty message is displayed, the amount of toner sucked by the screw pump is little. The quantity of heat discharged from the screw pump together with the toner decreases. If the toner supplying operation is performed by the screw pump similarly to the case where a sufficient amount of toner is present in the toner container, the internal temperature of the screw pump increases.
When the toner is a low temperature fixable toner, a problem in that the toner is melted in the screw pump tends to occur. When the toner is melted and then the internal temperature of the screw pump decreases after the screw pump is stopped, the toner is fixed to the stator and rotor. In this case, when the screw pump is driven again, the stator (typically made of an elastic material) is easily abraded by the toner, which is fixed to the contact portion between the rotor and the stator and which serves as an abrasive. Thereby, the sealing property of the screw pump is deteriorated due to the abraded portion of the stator, resulting in deterioration of the suction power of the screw pump.
This problem is not specific to such screw pumps, and may occur in any pumps in which a first member is moved while rubbing a second member contacted with the first member to generate a negative pressure at the suction opening thereof.
Because of these reasons, a need exists for a toner supplying method (or an image forming apparatus having a toner supplying device), which does not cause the above-mentioned problem in that the suction power of a pump (such as screw pumps) is deteriorated due to fixation of a low temperature fixable toner to the pump.